The present invention relates to a wireless base station, a wireless frame synchronization detection method used therein, and a recording medium on which a program therefor is recorded and, more particularly, to a wireless frame synchronization detection method of detecting wireless frame synchronization in a wireless base station (node-B) which performs communication by using the CDMA (Code Division Multiple Access) method.
The conventional general wireless frame synchronization method in node-B described in “W-CDMA (Wideband Code Division Multiple Access) Mobile Communication Method” (supervised by Keiji Tatekawa, and issued on Jun. 25, 2001 by Maruzen Kabushiki Kaisha)” will be described below.
The CDMA method describes the definitions of a transmission channel and physical channel, and the explanation of the definitions. The physical channel normally has a hierarchical arrangement including wireless frames and time slots, and the forms of the wireless frames and time slots change in accordance with the symbol rate of the physical channel.
The wireless frame is made up of 15 time slots, and is a minimum unit of signal processing. The time slot is a minimum constituting unit of a layer 1-bit sequence, and is a minimum processing unit of transmit power control and a channel estimation process. The number of bits in one time slot depends upon the physical channel.
Of the physical channels described above, in an uplink DPCH (Dedicated Physical Channel), two types, i.e., a DPDCH (Dedicated Physical Data Channel) used for data transmission and a DPCCH (Dedicated Physical Control Channel) used to transmit physical control information are multiplexed by I/Q [In-phase/Quadrature] for each wireless frame.
The DPCCH for handling control information is made up of pilot bits (Pilot) having a known pattern used for estimation in synchronization detection, a transmit power control command (TPC: Transmit Power Control), feedback information (FBI: Feedback Information), and a TFCI (Transport Format Combination Indicator).
FIG. 9 shows the wireless frame arrangement of the uplink DPCCH described above. Referring to FIG. 9, each wireless frame (10 ms) is divided into 15 slots, and one slot has 2,560 chips. The number of bits per slot of the uplink DPDCH/DPCCH is determined by a parameter k, and the parameter k corresponds to SF (Spreading Factor)=256/2 k of the physical channel. The SF of the DPDCH is set within the range of 256 to 4, and 256 (a fixed value) is set as the SF of the DPCCH. A slot format to be used in the DPCCH is determined by the use/nonuse of the TFCI, the use (the number of bits used)/nonuse of the FGI, and the application (the number of transmission slots)/non-application of a compression mode.
The CDMA method performs channel estimation by using the pilot bits, and detects frame synchronization by using an SW (Sync Word) contained in the pilot bits. As shown in FIG. 10, the conventionally general wireless frame synchronization detection method detects wireless frame synchronization establishment and synchronization pull out by using the correlation characteristics of the SW.
That is, in this wireless frame synchronization detection method, the pilot bits of the uplink DPCCH received in node-B are compared with a reference pilot bit pattern used for channel estimation, and, if the number of mismatch bits is equal to or smaller than a preset number of pilot error allowable bits, it is determined that pilot bit reception is OK.
Also, in the wireless frame synchronization detection method, if this pilot bit OK state continues for a predetermined frame period (a critical value used in this determination is called the number of frame synchronization backward protection steps), it is determined that wireless frame synchronization establishment is detected, and, if the pilot bit reception NG state continues for the predetermined frame period (a critical value used in this determination is called the number of frame synchronization forward protection steps), it is determined that wireless frame synchronization pull out is detected.
Referring to FIG. 10, in the above wireless frame synchronization detection method, after the start of synchronization establishment, pilot bit reception OK detection is started from wireless frame synchronization state=initial state (A), and, if a wireless frame period in which pilot bit reception is OK continues and becomes equal to or larger than the critical value: the number of frame synchronization backward protection steps, the process advances to wireless frame synchronization state=synchronization establishment (B) (a in FIG. 10).
Also, in the wireless frame synchronization detection method, pilot bit reception NG detection is started from wireless frame synchronization state=synchronization establishment (B), and, if a wireless frame period in which pilot bit reception is NG continues and becomes equal to or larger than the critical value: the number of frame synchronization forward protection steps, the process advances to wireless frame synchronization state=synchronization pull out (C) (b in FIG. 10).
In addition, in the wireless frame synchronization detection method, pilot bit reception OK detection is started from wireless frame synchronization state=synchronization pull out (C), and, if a wireless frame period in which pilot bit reception is OK continues and becomes equal to or larger than the critical value: the number of frame synchronization backward protection steps, the process advances to wireless frame synchronization state=synchronization establishment (B) (c in FIG. 10).
Furthermore, in the wireless frame synchronization detection method, a call is released from wireless frame synchronization state=synchronization establishment (B), or from wireless frame synchronization state=synchronization pull out (C), and the process advances to wireless frame synchronization state=initial state (A) (d in FIG. 10).
In the conventional general wireless frame synchronization detection method described above, however, the pilot error allowable bits and the number of frame synchronization protection steps used in the method are arbitrarily set for each node-B, so the standards for wireless frame synchronization establishment detection and synchronization pull out detection change in accordance with an arbitrary combination of a UE (User Equipment: mobile station) and node-B.
Accordingly, unified standards for wireless frame synchronization establishment detection, synchronization pull out detection, and synchronization maintenance detection are required even in an arbitrary UE and node-B, and a method of determining wireless frame synchronization by a more accurate method is being studied.
Also, in the conventional wireless frame synchronization determination method, wireless frame synchronization establishment may be detected by error in a wireless frame even if no upward signal is received. This is presumably caused by the following mechanism.
In a path capture process, a plurality of paths may be notified to a channel estimation means, even if there is no signal, depending on the set value of a path detection threshold. If channel estimation is performed for each path and phase correction is performed for the path by using a carrier wave phase having the highest correlation to the pilot bit pattern, a value closer to the pilot bit pattern than that when the output value of each path is random may be obtained.
Furthermore, if pilot bits output by RAKE combination of the value are equal to or smaller than the pilot error allowable bits, a synchronization establishment error occurs in a wireless frame.
This phenomenon occurs extremely generally not only in the CDMA system but also in any wireless system in which phase estimation is performed using the pilot bits, and wireless frame synchronization is determined using the pilot bit pattern after RAKE combination.
The foregoing is also obvious from the results of the simple simulation described below, and it is possible to confirm that a synchronization establishment error surely occurs by the above mechanism. The conditions of individual items in the channel estimation means used during the simulation are as shown in FIG. 11.
White noise is used as an input signal, and the weighted mean of the channel estimation values of two forward time slots and two backward time slots (a total of five time slots) is calculated, thereby estimating an FV (Fading Vector). This FV is used to perform phase correction and RAKE combination for the input signal, the degree of matching between an FSW (Frame Synchronous Word) contained in the signal and a transmission pattern is checked, and it is found that the FSW is four symbols out of six symbols of the pilot bit pattern per slot, and a total of 60 symbols are contained in one wireless frame.
FIG. 12 shows the simulation results plotted as a histogram. When no phase correction is performed, a wide distribution centering around 30 symbols is obtained as expected. When phase correction is performed, the center is around 40 symbols even when the number of captured paths is 1 (unit: path), and the degree of matching increases after that as the number of captured paths increases. When the number of captured paths is 10 (unit: path), the degree of matching reaches 56 or 57 symbols.